I. Pressure Swing Adsorption
Gas separation by pressure swing adsorption is achieved by coordinated pressure cycling and flow reversals over an adsorber contacted with a gas mixture that preferentially adsorbs a more readily adsorbed component relative to a less readily adsorbed component of the mixture. The total pressure is elevated during intervals of flow in a first direction through the adsorber from a first end to a second end, and is reduced during intervals of flow in the reverse direction. As the cycle is repeated, the less readily adsorbed component is concentrated in the first direction, while the more readily adsorbed component is concentrated in the reverse direction.
A first, or “light,” product, depleted in the more readily adsorbed component and enriched in the less readily adsorbed component, is delivered from a second end of the adsorber. A second, or “heavy,” product, enriched in the more strongly adsorbed component, is exhausted from a first end of the adsorber. The light product usually is the product desired to be purified by separation from the remaining components of the gas mixture, and the heavy product often is a waste or secondary product, as in the important examples of oxygen separation over nitrogen-selective zeolite adsorbents and hydrogen purification. The heavy product (enriched in nitrogen as the more readily adsorbed component) is a desired product in the example of nitrogen separation over nitrogen-selective zeolite adsorbents.
Typically, a feed mixture is admitted to a first end of an adsorber and the light product is delivered from the second end of the adsorber when the pressure is elevated to a higher working pressure. The heavy product is exhausted from the first end of the adsorber at a lower working pressure. To obtain a highly pure light product, a fraction of the light product enriched in the less readily adsorbed component is recycled back to the adsorbers as “light reflux” gas after pressure letdown, e.g. to perform purge, pressure equalization or repressurization steps.
The conventional process for gas separation by pressure swing adsorption uses two or more adsorbers in parallel, with directional valving at each end of each adsorber to connect the adsorbers in alternating sequence to pressure sources and sinks, thus establishing the changes of working pressure and flow direction. The basic pressure swing adsorption process inefficiently uses applied energy, because of irreversible expansion over the valves while switching the adsorbers between higher and lower pressures. More complex conventional pressure swing adsorption devices achieve some improvement in efficiency by using multiple “light reflux” steps, both to achieve some energy recovery by pressure equalization, and also desirably to sequence the light reflux steps so that lower purity light reflux gas reenters the second end of the adsorbers first, and higher purity light reflux gas reenters the second end of the adsorbers last, so as to maintain the correct ordering of the concentration profile in the adsorbers.
The conventional method of supporting the adsorbent is also problematic. There is a need for rigid high surface area adsorbent supports that can overcome the limitations of granular adsorbent and enable much higher cycle frequencies. High-surface-area laminated adsorbers, with the adsorbent supported in thin sheets separated by spacers to define flow channels between adjacent sheets, formed either as stacked assemblies or as spiral rolls, have been disclosed by Keefer, U.S. Pat. Nos. 4,968,329 and 5,082,473, which are incorporated herein by reference.
U.S. Pat. No. 4,968,329 discloses related gas separation devices with valve logic means to provide large exchanges of fresh feed gas for depleted feed gas. Such large feed exchanges may be required when concentrating one component as a desired product without excessively concentrating or accumulating other components, as in concentrating oxygen from feed air containing water vapor whose excessive concentration and accumulation would deactivate the adsorbent.
Siggelin (U.S. Pat. No. 3,176,446), Mattia (U.S. Pat. No. 4,452,612), Davidson and Lywood (U.S. Pat. No. 4,758,253), Boudet et al. (U.S. Pat. No. 5,133,784), and Petit et al. (U.S. Pat. No. 5,441,559) disclose PSA devices using rotary adsorber configurations. Ports for multiple, angularly separated adsorbers mounted on a rotor assembly sweep past fixed ports for feed admission, product delivery and pressure equalization. In this apparatus, the relative rotation of the ports provides the function of a rotary distributor valve. All of these prior art devices use multiple adsorbers operating sequentially on the same cycle, with multiport distributor rotary valves for controlling gas flows to, from and between the adsorbers.
The prior art includes numerous examples of pressure swing adsorption and vacuum swing adsorption devices with three adsorbers operating in parallel. Thus, Hay (U.S. Pat. No. 4,969,935) and Kumar et al. (U.S. Pat. No. 5,328,503) disclose vacuum adsorption systems which do not achieve continuous operation of compressors and vacuum pumps connected at all times to one of the three adsorbers. Such operation is achieved in other three adsorber examples provided by Tagawa et al. (U.S. Pat. No. 4,781,735), Hay (U.S. Pat. No. 5,246,676), and Watson et al. (U.S. Pat. No. 5,411,528), but in each of these latter examples there is some undesirable inversion of the ordering of light product withdrawal and light reflux steps so that process efficiency is compromised.
II. Fuel Cells
Various fuel cell types are known such as polymer electrolyte membrane (PEM) fuel cells, alkaline fuel cells and solid oxide fuel cells. In general, electrochemical fuel cells include an anode, a cathode, and an electrolyte, and generate electrical energy by converting chemical energy derived from a fuel directly into electrical energy by oxidizing fuel in the cell. Fuel and oxidant are supplied to the anode and cathode, respectively. In the case of PEM fuel cells, fuel permeates the electrode materials and reacts at the anode catalyst layer to form cations. These cations migrate through the electrolyte to the cathode. An oxidizing gas, such as oxygen or an oxygen-containing gas, supplied to the fuel cell reacts at the cathode catalyst layer to form anions, which react with the cations to form a reaction product. The fuel cell generates a useable electric current and the reaction product is removed from the cell.
If hydrogen is the fuel and oxygen-containing air (or pure oxygen) is the oxidant, a catalyzed reaction at the anode produces hydrogen cations from the fuel supply. An ion exchange membrane (1) facilitates the migration of hydrogen ions from the anode to the cathode, and (2) isolates the hydrogen fuel stream from the oxygen stream. At the cathode, oxygen reacts to form anions, which react with the hydrogen ions that have migrated across the membrane to form water as a reaction product.
The anode and cathode reactions for polymer-electrolyte-membrane-type fuel cells are shown in equations (1) and (2) below.Anode reaction H2→2H++2e−  Equation 1Cathode reaction {fraction (1/20)}2+2H++2e−→H2 O(2)  Equation 2
Two or more fuel cells connected together in series or in parallel are referred to as a stack and are used to increase the overall power output of the assembly. Fuel cells typically are connected in series with one side of a given plate serving as an anode plate for one cell and the other side of the plate serving as the cathode plate for the adjacent cell. The stack typically includes feed manifolds or inlets for directing fuel, such as substantially pure hydrogen, methanol reformate or natural gas reformate, and the oxidant, such as substantially pure oxygen or oxygen containing air, to the anode and cathode. The stack also generally includes exhaust manifolds or outlets for expelling any unreacted fuel and oxidant gases, each carrying entrained water, as well as an outlet manifold for the coolant water exiting the stack.
Fuel cells are known in the patent literature. For example, U.S. Pat. No. 5,200,278, entitled “Integrated Fuel Cell Power Generation System,” assigned to Ballard Power Systems, Inc., and incorporated herein by reference, describes one embodiment of a fuel cell useful as a power source. Fuel cells also are commercially available.
III. Portable Gas Separators
Portable gas separators, such as might be used for generating oxygen of sufficient purity for respiration by mammals, such as humans, such as greater than 90 volume % pure, and perhaps 95 volume % and higher, are inefficient and typically are not easily portable by an individual. As such, the uses of conventional separators are limited. A need therefore exists for a portable, efficient gas separator for use as, for example, a medical oxygen generator.